Laws Of Thermodynamics

[8.1] System Energy(Usys) and the Work Energy Principle

The work-energy principle states that an increase in the kinetic energy of a rigid body is caused by an equal amount of positive work done on the body by the resultant force acting on that body. Conversely, a decrease in kinetic energy is caused by an equal amount of negative work done by the resultant force.

∆Usys = W

Examples,

(1) A car is accelerating constantly. Here velocity is increasing. As a result of, kinetic energy is also increasing. So positive work is done.

(2) A ball is dropped from a certain height. Here velocity of the ball is increasing gradually due to gravitational acceleration. As a result of which kinetic energy increases and positive work is done.

(3) A motor bike applies break and comes into rest after few second. Here work done by the break is negative. Because speed decreases, for which K.E also decreases.

[8.1.1] Alternative Notation for Work Done

In the above mathematical relationship for the work-energy principle, work done on the system has taken as positive. However, this sign convention is only a matter of choice. The positive work directly corresponds to an increase in the system's capacity to do work, and therefore the system's internal energy.

However, if the work done by the system thought as positive, then a positive work will correspond to a decrease in the system's internal energy. Therefore in this convention, the work-energy principle will take the form,

∆Usys = −W

[8.2] Heat Transfer and the Change in the System's Energy

An amount of heat Q, given to a system will increase its ability to do work, and therefore its internal energy. Similarly, heat taken out of a system (−Q) will decrease the system's internal energy, such that

∆Usys = Q

[8.3] First Law of Thermodynamics

The first law of thermodynamics is a version of the law of conservation of energy, adapted for thermodynamic processes, distinguishing two kinds of transfer of energy, as heat and as thermodynamic work, and relating them to a function of a body's state, called Internal energy.

The first law of thermodynamics states that the change in internal energy of a system(U), equals the net heat transfer into the system(Q), plus the net work done on the system(W).

[8.4] Heat Capacity of a Gas System

Heat capacity or thermal capacity is a physical property of matter, defined as the amount of heat to be supplied to a given mass of a material to produce a unit change in its temperature. The SI unit of heat capacity is joule per kelvin (J/K).